1. Field of the Invention
This invention pertains generally to photosensors, and more particularly to a photosensor that incorporates high quantum energy photocathodes operated in reflection mode.
2. Description of the Background Art
Research in the area of high-energy physics and high-energy astrophysics is often based on the detection of Cherenkov or fluorescence photons emitted by charged particles in a wide variety of transparent media. Efficient photon detection is a key element that is common to this work.
At present, conventional PhotoMultiplier Tubes (PMTs) are used in most of these applications because they combine affordability with good timing properties. However, PMTs tend to suffer from poor single-photon resolution and low quantum efficiency. Furthermore, PMTs based on reflective photocathodes have reached a very limited application, mainly because of their large dead area and non-uniform angular acceptance.
Hybrid Photon Detectors (HPDs) provide excellent single-photon resolution, but by) fail to offer higher quantum efficiency because they tend to be based on the same photocathode materials as PMTs. On the other hand, HPDs housing GaAs and GaAsP photocathodes in transmission mode have been recently developed that offer peak quantum efficiency of up to approximately 50%. However, due to the complicated Molecular Beam Epitaxy (MBE) process used in manufacturing, these HPDs have been very expensive. In addition, they suffer from a large dead area and difficulties in the formation of compound hexagonally packed imaging cameras. An imaging HPD based on a reflective photocathode formed in a bend structure has been developed as well, but has been relatively difficult to manufacture and suffers from narrow angular acceptance and poor resolution in response time.
In a typical transmission-type photocathode, photons enter the photocathode layer from one side, while the photoelectrons emerge into vacuum on the opposite side of the layer. This concept is far from being optimal since the electrons have to be emitted at a surface that is far from the place of their most abundant creation, i.e., the surface on the opposite side of the layer. Electrons have a relatively low chance to diffuse from the region close to the photon entry surface to the surface on the opposite side. The design of a transmission photocathode has therefore always been a compromise between two conflicting requirements: (a) efficient photon conversion and (b) successful electron diffusion to the surface.
The situation is fundamentally different in reflection-type photocathodes since the electrons are emitted through the same surface the photons have entered. The majority of electron-hole pairs are created very close to the photon entrance surface (due to the Lambert-Beer exponential law) and therefore have a high chance of reaching the same surface and escaping though it into vacuum. As a result, reflection cathodes offer quantum efficiency nearly twice that of transmission photocathodes. The sensitivity to ultraviolet (UV) light is enhanced even more, since the short wavelength photons are absorbed closer to the surface.
Apart from a considerable increase in quantum efficiency and an important widening of the spectral response into the short wavelength range, reflection photocathodes offer other very important advantages. The most important advantage is the significant simplification of the photocathode manufacturing process, and a consequent price reduction. This general feature in particular concerns the most efficient but extremely expensive III-V semiconductor photocathodes (e.g., GaAs, GaAsP, InGaAs, etc.) processed by MBE expitaxial growth in an ultra high vacuum. A typical production process for transmission-type III-V photocathodes consists of approximately ten different steps, starting from expitaxial growth of a thin photocathode layer on top of a crystal substrate with matched lattices constant, fusion of the grown structure to the photube entrance window with the help of previously MBE-deposited additional interface layers, and finally removal of the growth substrate from the opposite side of the photocathode layer.
In contrast, the production of a reflection-type III-V photocathode is much simpler since there is no need to fuse the grown photocathode structure with the glass window and to remove the growth substrate. This leads to a very significant cost reduction that is likely to bring the III-V photocathodes into an affordable price range, with unprecedented high quantum efficiency (e.g., for GaAsP, approximately two to three times higher than that of transmission bialkalai photocathodes). In addition, while for a typical transmission-type photocathode a thick conductive sub-layer has to be deposited between the glass window and the photocathode, in a reflection-type photocathode the thickness and the optical properties of this conductive sub-layer are not critical since photons do not need to pass through. Although only around twenty atomic layers thin when used with transmission photocathodes, this sub-layer (e.g., SnO or indium-tin-oxide) absorbs about 25% of the incoming light, which presents a significant loss even before the light has reached the transmission photocathode. In contrast, reflection photocathodes may even benefit from the conductive layer underneath, since it may serve as a mirror that reflects transmitted light back through the photocathode layer, thus providing the photon with another conversion opportunity.
In spite of these striking advantages, reflection-type photocathodes have never had a wide application in photosensor devices due to the lack of a phototube design that would simultaneously host a photocathode in a reflection configuration, and provide the following important features: (a) negligible dead area; (b) flat angular acceptance and sharp angular cutoff for detected light, (c) fast and position-independent time response, and (d) the possibility of close packing of individual units into large-area multi-pixel honeycomb imaging cameras.
Accordingly, there has been a strong need for a new kind of photosensor that would be able to incorporate photocathodes of the highest quantum efficiency at a relatively low cost. Such photosensors may replace conventional PMTs in many applications (e.g., physics, astronomy, industry, medicine) where high quantum efficiency, single-photon resolution, negligible dead area and other important properties are required. Among the different photocathode types, epitaxially grown III-V photocathodes (such as GaAsP, GaAs, and InGaAs) provide the highest possible quantum efficiency. As stated previously, for example, the quantum efficiency of GaAsP photocathodes may reach almost fifty percent.
Single-photon sensitivity, single-photon resolution, excellent time resolution, low noise, and, most important, high quantum efficiency (e.g.,  greater than 50%), are key features of an ideal, but so far nonexistent photosensor. A photosensor comprising all of these qualities at a low price would open a new range of sensitivity in different frontiers, such as photon decay, neutrino oscillations, neutrino astronomy, gamma-ray astronomy, and the like. The present invention satisfies those requirements, as well as others, and overcomes deficiencies found in conventional photosensors.
The present invention generally comprises a photocathode operating in the so-called xe2x80x9creflection modexe2x80x9d instead of the traditional transmission mode. By way of example, and not of limitation, a photosensor according to the present invention combines reflective mode photocathode technology with compound parabolic light concentrator (CPC) technology wherein the same vacuum tube components act both as an incoming light concentrator and as a focusing electron lens. The interior of the CPC is electrically conductive and split into two electrodes by a non-conductive interval, such that photoelectrons emitted by the photocathode are electrostatically focused by the same CPC-shape onto a small light collection surface at entrance end of the photosensor.
Reflection photocathodes offer much higher quantum efficiency with the same photocathode material. However, they have never been widely used due to the lack of a photosensor configuration that would be able to host the reflection photocathode without significant sacrifices in effective sensitive area, photon angular acceptance, and time resolution. The photosensor configuration of the present invention overcomes those problems, facilitates the long-awaited significant increase in quantum efficiency, and provides further improvements in overall design and operational characteristics.
According to an aspect of the invention, type III-V material is used for the reflective mode photocathode since the manufacturing process is radically simpler and less expensive than the manufacturing process of a photocathode in transmission mode. In contrast to the production of photosensors hosting transmission photocathodes, the production of a photosensor based on the present invention requires neither the fusion of the epitaxially grown photocathode surface with the entrance window of the photosensor, nor the removal of the thick epitaxial growth substrate from the opposite side. Apart from that, the intrinsic quantum efficiency is higher for reflective-mode photocathodes.
In accordance with another aspect of the invention, the same vacuum tube components act both as a perfect incoming light concentrator and as a perfect focusing electron lens. The former assures that all the incoming photons will reach the photocathode, and the latter enables all of the emitted photoelectrons to hit the semiconductor sensor.
The present invention is based on the discovery that Compound Parabolic light Concentrators (CPCs or Winston cones), known to provide the most efficient non-imaging light concentration may simultaneously act as ideal focusing lenses. While the light is focused in one direction along the axis of a cylindrically symmetric CPC (i.e., from the large-area entrance to the small-area light collection surface), in the present invention the photoelectrons are electrostatically focused by the same CPC-shape in the opposite direction (i.e., from the small light collection surface towards a point-like region in the middle of the large-area entrance aperture).
Therefore, in accordance with the present invention a CPC must be electrically conductive. In accordance with another aspect of the invention, the electrically conductive CPC is split into two electrodes by a narrow non-conductive interval positioned in a particular place along the CPC. In the present invention, the photocathode covers the light collection area of a CPC, which assures that essentially all the light accepted by the CPC is collected on the photocathode surface. Since the CPC will concentrate light in the most efficient way, this configuration provides the best possible usage of the photocathode surface. The photocathode is operated in the reflective mode; that is, the photoelectrons emerge from the same surface through which the photons enter. Photoelectrons emerging from the entire photocathode are accelerated and focused onto a small electronic sensor placed in the middle of the entrance aperture of the CPC.
An object of the invention is to provide the highest possible quantum efficiency at a low manufacturing cost.
Another object of the invention is to provide for optimal usage of the photocathode surface.
Another object of the invention is to provide a negligible dead area.
Another object of the invention is to provide for flat angular acceptance for incoming light.
Another object of the invention is to provide for sharp angular cutoff for incoming light at a required incidence angle.
Another object of the invention is to provide for the possibility of modifying the lateral shape of the entrance section of the phototube from circular to hexagonal, without degradation of performance.
Another object of the invention is to provide for hexagonal honeycomb close packing of photosensors into large-area imaging cameras.
Another object of the invention is to eliminate the need for additional light concentrators or stray light protection.
Another object of the invention is to provide for fast and position-independent time response.
Another object of the invention is to provide single-photon resolution.
Another object of the invention is to provide for color sensitivity without destructive filtering.
Another object of the invention is to provide for extended spectral sensitivity.
Further objects and advantages of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon.